1. Field of the Invention
The present invention relates to a clock synchronizing apparatus for synchronizing the clock of a closed communication network such as a LAN or a MAN with the clock of a wide area communication network, a network therewith, and a clock synchronizing method thereof.
2. Description of the Related Art
In recent years, as optical communications have become common, high speed digital transmissions have been employed for closed networks such as LANs and MANs. When a digital communication is made from a closed network to an external wide area digital network, the clock of the closed network should be synchronized with the clock of the external wide area digital network. Normally, since the closed network is connected to the wide area network as a subordinate system, the clock of the closed network is subordinately synchronized with the clock of the wide area network. In this case, clock synchronizing apparatuses should be disposed at a plurality of nodes of the closed network to maintain the reliability.
Even if clock synchronizing apparatuses are disposed at a plurality of nodes, the clock of the closed network is synchronized with the clock of the wide area network momentarily at only one node. When the clock synchronizing apparatus at the node fails to synchronize the clock with the clock of the wide area network, this clock synchronizing apparatus is switched to another clock synchronizing apparatus. At this point, complicated control signals should be exchanged between these clock synchronizing apparatuses. Thus, while the clock synchronizing apparatuses are being switched, a trouble such as loss-of-Synchronization or out-of-synchronization may take place.
If the clocks of clock synchronizing apparatuses at a plurality of nodes can be synchronized with the clock of the wide area network, even if a synchronous input signal stops at one node, the network system is less affected. Thus, such a problem can be prevented. This technique is referred to as a hybrid synchronization. The hybrid synchronization causes the wide area network to subordinately synchronize the clock with the clocks at nodes in the closed network and each of the nodes to mutually synchronize the clock with those of the other nodes.
However, the operation of the hybrid synchronization has not been satisfactorily studied. Thus, details such as constants of the hybrid synchronization are unknown. Consequently, the hybrid synchronization has not been widely used although its advantages are well known.